The present invention relates to a manufacturing method for a magnetic head, and more particularly to a manufacturing method for a magnetic head using a magnetic core formed of monocrystal ferrite or combined monocrystal ferrite and polycrystal ferrite.
A magnetic head to be mounted in a magnetic recording and reproducing device such as a VTR (video tape recorder) is constituted of a coil and a magnetic core formed of a magnetic material such as monocrystal ferrite. Such a magnetic head using a magnetic core formed of ferrite is called a ferrite head, and it is generally used in the art.
FIG. 25 shows a structure of such a magnetic head in the related art. Referring to FIG. 25, reference numerals 207 and 208 designate a pair of magnetic cores bonded to each other. The magnetic cores 207 and 208 are formed with track width defining grooves 201 and 202, respectively, for defining a track width. The magnetic core 207 has a front gap forming surface 203 and a back gap forming surface 205, and the magnetic core 208 has a front gap forming surface 204 and a back gap forming surface 206. The front gap forming surface 203 of the magnetic core 207 faces the front gap forming surface 204 of the magnetic core 208 to form a front gap g.sub.3 therebetween. Similarly, the back gap forming surface 205 of the magnetic core 207 faces the back gap forming surface 206 of the magnetic core 208 to form a back gap g.sub.4 therebetween. Further, the grooves 201 of the magnetic core 207 face the grooves 202 of the magnetic core 208. Gap films 209 and 210 are formed on the opposed surfaces of the magnetic cores 207 and 208, respectively. A fusing glass 211 as a nonmagnetic material is filled in the vicinity of the front gap g.sub.3 and the back gap g.sub.4 to bond both the magnetic cores 207 and 208 each other. Further, the opposed surfaces of the magnetic cores 207 and 208 between the front gap g.sub.3 and the back gap g.sub.4 are formed with coil grooves 212 and 213 for receiving coils, respectively.
The magnetic head shown in FIG. 25 is manufactured by the following method. Firsts as shown in FIG. 26, a substrate 214 formed of monocrystal ferrite or combined monocrystal ferrite and polycrystal ferrite is prepared, and a plurality of track width defining grooves 215 for defining a track width of the magnetic head are formed on an upper surface of the substrate 214 so as to be arranged at a given pitch in a lateral direction of the substrate 214 and extend in a longitudinal direction of the substrate 214. Each groove 215 has a substantially semi-circular cross section.
Then, as shown in FIG. 27, coil grooves 216 and 217 for receiving coils and glass grooves 218 and 219 for receiving glass are formed on the upper surface of the substrate 214 so as to be arranged at a given pitch in the longitudinal direction of the substrate 214 and extend in the lateral direction of the substrate 214. Each of the coil grooves 216 and 217 has a substantially trapezoidal cross section, and each of the glass grooves 218 and 219 has a substantially U-shaped cross section. Thus, the coil grooves 216 and 217 and the glass grooves 218 and 219 extend in orthogonal relationship to the track width defining grooves 215 on the upper surface of the substrate 214.
Then, the substrate 214 is divided into a block 220 having the coil groove 216 and the glass groove 218 as shown in FIG. 28 and a block 221 having the coil groove 217 and the glass groove 219 as shown in FIG. 29.
Then, as shown in FIG. 28, a gap film 222 of a nonmagnetic material such as SiO.sub.2 is formed on the upper surface of the block 220 except the inner surfaces of the coil grooves 216 and the glass grooves 218 so as to have a thickness about half a gap length of the magnetic head by using a suitable thin film forming technique such as a magnetron sputtering process, thus forming a plurality of front gap forming surfaces 223 and a plurality of back gap forming surfaces 225. Similarly, a gap film 227 is formed on the upper surface of the block 221 to form a plurality of front gap forming surfaces 224 and a plurality of back gap forming surfaces 226 (see FIG. 29).
Then, as shown in FIG. 29, both the blocks 220 and 221 are matched with each other so that the front gap forming surfaces 223 and 224 face each other and the back gap forming surfaces 225 and 226 face each other.
Then, glass rods formed of a fusing glass are inserted into a space defined by the coil grooves 216 and 217 and a space defined by the glass grooves 218 and 219, and the glass rods are then fused at a given temperature under a pressure of about tens of MPa applied in opposite directions depicted by arrows P.sub.2 in FIG. 29.
Accordingly, a fusing glass 231 is filled in the vicinity of a plurality of front gaps defined between the front gap forming surfaces 223 and 224, in the vicinity of a plurality of back gaps defined between the back gap forming surfaces 225 and 226, and in a plurality of spaces defined by the track width defining grooves 215 and 230. Thus, the front gaps and the back gaps functioning as recording and reproducing gaps are formed between the front gap forming surfaces 223 and 224 and between the back gap forming surfaces 225 and 226, respectively.
Then, a tape sliding surface 232 of the combined blocks 220 and 221 is subjected to cylindrical grinding.
Finally, the integrated body of the blocks 220 and 221 is cut into chips, and coils are located in the coil grooves 216 and 217 of each chip, thereby obtaining the magnetic head shown in FIG. 25.
However, there occur various strains in the monocrystal ferrite in the above manufacturing process. The strains cause a great reduction in permeability of the magnetic cores to reduce a reproduction efficiency of the magnetic head.
In the magnetic head to be manufactured by the above method for example, the pair of blocks 220 and 221 forming the pair of magnetic cores 207 and 208 are bonded together by matching the blocks 220 and 221 and fusing the fusing glass 231 as a nonmagnetic material with heat and pressure to fill the fusing glass 231 between the blocks 220 and 221. In most cases, the blocks 220 and 221 before matching are curved. Accordingly, if the blocks 220 and 221 are merely matched with each other, full-face contact between the front gap forming surfaces 223 and 224 and full-face contact between the back gap forming surfaces 225 and 226 cannot be obtained to cause opening of the magnetic gap. To cope with this problem, a given pressure (e.g., about tens of MPa) is applied to the blocks 220 and 221 in heating the fusing glass 231 and filling it between the blocks 220 and 221, thereby eliminating the curvature of the blocks 220 and 221 to form a desired magnetic gap.
However, this pressure is concentrated at the magnetic gap forming portion formed of monocrystal ferrite or combined monocrystal ferrite and polycrystal ferrite as a magnetic material for forming the blocks 220 and 221. As a result, even after bonding the blocks 220 and 221, an internal residual stress exists in the ferrite forming the magnetic gap forming portion. Therefore, a magneto-mechanical coupling effect due to the residual stress causes a reduction in permeability of the magnetic cores to reduce the reproduction efficiency of the magnetic head.
In recent years, it has been demanded to reduce a track width of the magnetic head in response to a demand for high-density recording. The reduction in the track width results in an increase in the stress applied to the ferrite forming the magnetic gap forming portion. Accordingly, a large strain occurs in the magnetic gap forming portion to reduce the permeability of the ferrite and reduce the reproduction efficiency of the magnetic head. In particular, when the track width is about 20 .mu.m or less, the above problem becomes remarkable.
Hitherto, there does not exist a method for quantitatively relating a reduction in permeability of a ferrite material due to a residual stress to a reduction in reproduction output of an actual magnetic head. Accordingly, it is necessary to actually make a magnetic head and measure a reproduction output of the magnetic head, so as to determine how the residual stress generating in forming the magnetic gap reduces the reproduction output. Thus, in the present circumstances, it is impossible to establish a definite guideline for designing in relation to the stress. Further, a stress dependency of permeability in monocrystal ferrite may change with a crystal orientation of the ferrite. That is, in designing a magnetic head employing monocrystal ferrite, it is necessary to decide a cutting face orientation of the monocrystal ferrite in consideration of an aspect of workability and wear resistance and another aspect of reproduction output and recording characteristic. It is further necessary to control the residual stress in the manufacturing process, so as to prevent a deterioration in magnetic characteristics of the ferrite. Thus, such a series of operations must be carried out by trial and error.